Current and Resistance
Chapter 17
Lesson 3
p. 608
Current and Charge Movement
• Electricity became an integral part of our daily
lives when scientists learned to control the
movement of electric charge, called current.
• Electric currents are even part of the human
body. (Luigi Galvani’s experiment)
• Electric currents are responsible for
transmitting messages between body muscles
and the brain. In fact, every function involving
the nervous system is initiated by electrical
activity.
Current
Electric Current
• Definition: the rate at which electric charges
pass through a given area.
• Formula:
Q
I
t
electric current = charge passing through a given area
time interval
• The SI unit for current is ampere.
• 1 A = 1 C/s
Electric Current
• Alternative Formula:
N (q e )
I
t
where:
N = number of electrons
qe  1.60 x10
19
C / electron
Word Problems (Practice C)
1. The current in a light bulb is 0.835 A. How
long does it take for a total charge of 1.67 C to
pass through the filament of the bulb?
(Sample Problem C, p. 609)
2. The compressor on an air conditioner draws
40.0 A when it starts up. If the start-up time is
0.50 s, how much charge passes a crosssectional area of the circuit in this time?
(Practice C, # 4, p.609)
Word Problems
3. A total charge of 9.0 mC passes through a
cross-sectional area of a nichrome wire in
3.5 s.
a. What is the current in the wire?
b. How many electrons pass through the
cross-sectional area in 10.0 s?
c. If the number of charges that pass through
the cross-sectional area during the given time
interval doubles, what is the resulting current?
(Practice C, # 5, p. 609)
Conventional Current
• Positive and negative charges in motion are
called charge carriers.
• Conventional current is defined in terms of the
flow of positive charges.
• For a material to be a good conductor, charge
carriers in the material must be able to move
easily through the material.
• Body fluids and salt water are able to conduct
electric charge because they contain charged
atoms called ions.
• An electrolyte is a solute that dissolves in water
to give a solution that conducts electric current.
Drift Velocity
• It is the net velocity
of a charge carrier
moving in an electric
field.
• Drift speeds are
relatively small.
Resistance to Current
• Definition: the opposition presented to
electric current by a material or device
• Formula:
V
R
I
resistance = potential difference
current
• The SI unit for resistance is ohm.
• 1Ω=1V/A
Ohm’s Law
• Resistance is constant over a range of
potential differences.
• Georg Simon Ohm (1789 – 1854), was the first
to conduct a systematic study of electrical
resistance.
• ∆V = IR
• Ohm’s law does not hold true for all materials.
(non-ohmic, e.g. diode)
Factors Affecting Resistance
1. Length – the longer, the greater resistance
2. Cross-sectional Area – the larger, the lesser
resistance
3. Material – conductors have lesser resistance
than insulators
4. Temperature – higher temperature means
greater resistance
Resistors
Resistors
• Resistors can be used to control the amount of
current in a conductor. This is often more
practical than changing the potential
difference or the properties of the conductor.
• Salt water and perspiration lower the body’s
resistance.
• Salt water contains ions that readily conduct
electric charge.
Resistors
• In a galvanic skin response (GSR) test,
commonly used as a stress test and as part
some so-called lie detectors, a very small
potential difference is set up across the body.
• In GSR tests, a state of low stress and high
resistance, or “normal” state, is used as a
control, and a state of higher stress is
reflected as a decreased resistance compared
with the normal state.
Resistors
A potentiometer is a
special type of resistor
that has a fixed contact
on one and an
adjustable, sliding
contact that allows the
user to tap off different
potential differences
Word Problems (Practice D)
1. A 1.5 V battery is connected to a small light
bulb with a resistance of 3.5 Ω. What is the
current in the bulb? (#1, p. 615)
2. The current in a microwave oven is 6.25 A. If
the resistance of the oven’s circuitry is 17.6 Ω,
what is the potential difference across the
oven? (# 4, p. 615)
3. A typical color television draws 2.5 A of
current when connected across a potential
difference of 115 V. What is the effective
resistance of the television set? (#5, p. 615)